A gaseous decomposition A(g) -> B(g) obeys first-order kinetics. Which curve correctly represents how the rate of formation (R) of B varies with time t?

R decreases exponentially with time, starting from a maximum at t = 0 and approaching zero
R increases exponentially with time without limit
R stays constant (a horizontal line) at all times
R increases linearly with time

Correct answer: R decreases exponentially with time, starting from a maximum at t = 0 and approaching zero

Solution

The rate of formation of B equals the rate of consumption of A, which is k[A]. Since [A] falls exponentially, R also falls exponentially from its initial maximum toward zero.

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